The invention relates to the design of a blade conveyer within a continuously operating digester of the Bauer M and D type, adapted for boiling processes particularly in the manufacture of cellulose.
The digester of the Bauer M and D type is a highly efficient arrangement designed for co-current continuous boiling processes. The digester proper is enclosed in a longitudinal cylindrical or tubular steel vessel, which is inclined with respect to a horizontal base plane. The internal space of the digester is divided into two parts by a partition wall passing through the longitudinal axis of the vessel--an upper part and a lower part. An endless blade conveyor is situated within the digester along its whole length, advancing in the upper part thereof downwards and in the lower part upwards. A supply port for the material to be treated is near the top of the vessel in its upper part, where the blade conveyer moves downwards. An outlet port for the treated material is near the top of the vessel in its lower part where the blade conveyer moves upwards.
The boiling liquid, for instance water or lye is supplied into the vessel by means of proper feeding means and the level of the boiling liquid is maintained constant in the course of operation. The level of the boiling liquid reaches as a rule up or close to the level of the outlet port for the treated material. The boiling liquid thus occupies both parts of the vessel of the digester, whereby part thereof leaves the digester together with the treated material over the outlet port.
The operating or boiling space between the blades of the advancing blade conveyor is filled through the supply port with the material to be treated by means of a continuously operating dosing device. The degree of filling of the working space by the material determines the maximum performance of the digester at the respective speed of the blade conveyor. The maximum degree of filling of the working space is about 85 percent. A lowering of the performance of the digester can be achieved by lowering the degree of filling of the working space with the material below 85 percent.
Investigations of technological properties of digesters of the Bauer M and D type showed that the boiling conditions in the digester can be in addition to other values characterized by two hydromodules, by a static and dynamic one. The static hydromodule can be defined as the ratio of the weight of the absolutely dry raw material supplied to the boiling space and of the weight of the boiling liquid, which both are simultaneously present in the working space of the digester. The static hydromodule has some similarity with the hydromodule of classical discontinuous digesters, where it characterises conditions when starting the process. The dynamic hydromodule can be defined as the ratio of the weight of the absolutely dry material and of the weight of the boiling liquid supplied to the digester within a certain time interval. The concentration of the liquor on the output from the digester depends on the value of the dynamic hydromodule.
It has been found by measurements and by calculations, that there are mutual relations between the degree of filling of the working space and the static and dynamic hydromodules. For instance in case of a maximum filling of the working space at 85 percent, i.e. at the maximum performance of 100 percent of the digester, the static hydromodule is 1:4.8. By reduction of filling of the digester the value of the static hydromodule increases within the digester, i.e. the ratio between the weight of the material and of the boiling liquid is reduced. At a degree of filling of 42.5 percent of the digester, i.e. at a 50 percent performance, the static hydromodule is already very high, about 1:10.8.
In case of a uniform advance of the treated material and of the boiling liquid with the same forward speed in the digester, the static and dynamic hydromodules are equal, as follows from the design of the digester. Optimum conditions of boiling processes however require a lower dynamic hydromodule, mostly within 1:2.0 to 1:2.5, possibly also up to 1:3.0. In order to obtain the required low hydromodule, a movement of the content of the digester of the Bauer M and D type has to be assured, where the advance of the material is quicker than the advance of the boiling liquid. Particular relations of flows of the boiling liquid in the digester are originating. If we consider the ratio of volumes of components in the digester, for instance in the case of an aqueous initial hydrolysis of beech chips, than in case of a filling of the digester to 85 percent at a static hydromodule equal to 1:4.8 and a dynamic hydromodule equal to 1:2.5, the advance speed of the chips in the digester is nearly twice the speed of the liquid. In case of a half performance of the digester, i.e. at a degree of filling 42.5 percent at a static hydromodule 1:10.8 and a dynamic hydromodule 1:2.5, the advance speed of the chips is already 4.3 times quicker than the advance speed of the liquid. These advance speeds correspond to a straight line flow parallel with the digester axis through cross sections which are proportional to the volume portion of materials in the digester. As however the liquid has at different speeds of the material and liquid only the possibility to flow over gaps between the blades of the conveyor and the internal wall of the vessel of the digester at the circumference of the blades, the maximum advance speed of the liquid is at places of the throughflow higher than considered and the difference of speed is still higher than above mentioned. A consequence of these particular conditions of liquid flow in the digester is the localisation of the intensive flow of the liquid along the circumference of the blades of the blade conveyor, taking along the material. On the other hand the flow of liquid through the material situated in the center of the blades is rather slight. The non-uniform flow of the boiling liquid causes a non-uniform diffusion process in the course of boiling, a non-uniform heat passage between the liquid and solid phase and the transfer of the granular material into the gap between the blades and the wall of the vessel of the digester, which material is rubbed off against this wall, whereby part of the granular material slides over into the following space between the blades and the time interval of its boiling is prolonged. These conditions of flow of the boiling liquid and their consequences can cause a certain irregularity of the boiling process and in case of cooperation of further conditions cause technological troubles.
When increasing the value of the dynamic hydromodule it is in principle possible to reduce the mentioned irregularities of flow of the boiling liquid, however a high dynamic hydromodule means an increased amount of boiling liquid, an increase of power consumption and reduced concentration of liquor, enhancing thus the rentability of the subsequent working or regeneration of liquor, so that this method of operation of the digester cannot be recommended for continuous operation.
This drawback of the digester is caused by the circumstance, that only a small gap is formed between the wall of the vessel of the digester and the circumference of the blades, so that a substantially closed space is created between adjacent blades, where the material together with the boiling liquid is at a certain ratio advanced toward the outlet port. At these conditions a low dynamic hydromodule can be obtained only in that the boiling liquid passes from one space between adjacent blades to the following one by way of gaps between the circumference of the blade and the wall of the digester vessel. Thus the already mentioned irregularity of flow is caused.